Clemens, in U.S. Pat. No. 3,842,194, discloses a capacitive video disc playback system which includes a high density information record having a conductive surface. Initially, information records for this system were of a layered construction, i.e. the record consisted of a plastic disc coated with a layer of conductive metal which was in turn coated with a layer of a dielectric material, such as polystyrene. Subsequently, information discs for this system were improved by replacing the layered construction with a thermoplastic matrix having finely divided carbon particles distributed therein. However, in order to make such improved discs sufficiently conductive to obtain capacitive playback, a fairly high loading of carbon black is required. Formulations presently utilized to prepare capacitive information discs contain about 15 percent by weight of a low density carbon black in a poly(vinyl chloride)-based resin. Suitable conductive molding compositions for these discs are disclosed by Martin et al. in U.S. Pat. No. 4,228,050. Such formulations contain, in addition to the two principal ingredients, a number of additives such as compatible lubricants, stabilizers, processing aids and the like.
Molding compositions, such as described by Martin et al., are comparatively expensive due principally to the amount of high quality, low density, conductive carbon black contained therein. The number of additives present also contributes to the cost of the disc as each must be separately produced or purchased, checked for purity, stored and the like. It would be beneficial to be able to significantly reduce the amount of carbon black and additives present in each disc without losing the conductivity necessary for capacitive playback of high quality video, audio and color signal information.
A potential means of reducing the carbon black content of a capacitive electronic disc would be to produce a disc having a nonconductive central core with conductive regions or layers on the outer surfaces.
Discs having a laminated structure are known in the audio record art. For example, audio records have been produced by laminating transparent plastic outer layers onto a picture mounted onto a central core. Audio records, of course, do not require a conductive outer surface. Japanese patent publications Nos. 53-119017 and 49-114412 suggest a sandwich-type capacitive electronic disc. These publications, however, are not enabling in that they do not disclose a suitable molding composition for such discs, or any specific process parameters of how a suitable disc might be formed.
O'Mara, in U.S. Pat. No. 4,390,487, discloses a method of forming a sandwich-type capacitive electronic disc in which the conductive plastic is injected into a compression mold and a nonconductive core material injected inside the conductive material to form a layered preform which is compression molded to form a multilayered capacitive electronic disc. Discs made by this process contain up to about 70 percent by weight of nonconductive material.
Ruda, in copending U.S. patent application Ser. No. 522,332, filed Aug. 11, 1983, discloses a sandwich-type capacitive electronic disc having conductive layers which are thin in comparison to those in a disc prepared according to O'Mara. In Ruda, the conductive plastic composition is formed into sheets about 2 to 30 mils thick by passing between rollers in commercial calendering equipment or in a two-roll mill. Sandwich discs are formed by compression molding a plastic core between two such sheets. The core is comprised of a conventional audio record formulation. The conductive composition is, in essence, that described by Martin et al.
In accordance with this invention, a method has been provided to prepare unique sandwich-type capacitive electronic discs which method is advantageous over previous methodologies.